The present inventor has previously proposed a method for highly hydrophilifying the surface of articles by the action of a photocatalyst (International Publication No. WO96/29375). According to this method, a coating of a semiconductor photocatalyst, such as the anatase form of titania, is provided on the surface of an article. Photoexcitation of this photocatalytic coating by exposure to light having a satisfactory intensity for a satisfactory period of time permits the surface of the photocatalytic coating to be highly hydrophilified to such an extent as will make a contact angle with water of about 0.degree..
As disclosed in WO96/29375, the above highly hydrophilifiable photocatalytic coating is applicable to various articles for antifogging, antifouling, selfcleaning and other various purposes. For example, when the photocatalytic coating is provided on a transparent article, such as a windshield for a vehicle, a windowpane for a building, or an eyeglass lens, or a mirror, the surface of the coating is highly hydrophilified upon photoexcitation of the photocatalyst, preventing the article from being fogged by moisture condensate or steam or from being blurred by water droplets adhering on the surface thereof. Further, when a building or an article, which is disposed outdoors, is provided with the photocatalytic coating, oil repellent or hydrophobic dust and contaminants adhering on the hydrophilified surface are washed away by raindrops every time they are exposed to rainfall, thus permitting the surface to be selfcleaned.
Various light sources capable of emitting light having higher energy than the band gap energy of a photocatalyst, a semiconductor, are utilized for the photoexcitation of the photocatalyst. In the case of a photocatalyst, of which the photoexcitation wavelength is in the ultraviolet region, such as titania, ultraviolet light is necessary for the photoexcitation of such a photocatalyst. In this case, when the article is in such a state as will be exposed to the sunlight, the photocatalyst can be advantageously photoexcited by ultraviolet light contained in the sunlight.
So far as the photoexcitation continues, the surface of the photocatalytic coating permanently retains its hydrophilic property. Interruption of the photoexcitation results in gradual decreased hydrophilicity of the surface of the photocatalytic coating. This is considered attributable to the fact that the surface of the photocatalytic coating is gradually contaminated with a hydrophobic material. As soon as the photocatalyst is photoexcited again, the hydrophilicity is recovered.
In the case of photoexcitation in a discontinuous manner like photoexcitation of the photocatalyst by the sunlight, the hydrophilicity of the surface of the photocatalytic coating is attenuated upon the interruption of the photoexcitation, while the resumption of the photoexcitation results in recovery of the hydrophilicity. Thus, the attenuation and the recovery of the hydrophilicity are alternately repeated.
A primary object of the present invention is to improve the above conventional method, enabling a surface to be hydrophilified more easily.
Another object of the present invention is to improve the above conventional method, enabling a surface to be highly hydrophilified upon exposure to weaker light, that is, lower irradiation intensity.
Still another object of the present invention is to provide a method which, even when photoexcitation is interrupted or when an article is placed in the dark, enables a high level of hydrophilicity imparted to a surface to be kept for as long a period of time as possible.
A further object of the present invention is to provide a method which, when photoexcitation is resumed after interruption of the photoexcitation, enables the hydrophilicity of a surface to be recovered upon exposure to light for a short period of time or upon exposure to weak light.
A still further object of the present invention is to provide a composite for use in practice of the above methods.